[layoutng] Rename struct members to match style guide

This was focused on ng_units and ng_margin_strut, though I also updated
some more variables in ng_block_layout_algorithm.

https://google.github.io/styleguide/cppguide.html#Variable_Names

R=ikilpatrick@chromium.org,eae@chromium.org
BUG=635619
NOTRY=true

Review-Url: https://codereview.chromium.org/2272313004
Cr-Commit-Position: refs/heads/master@{#414483}
11 files changed